Why does my outdoor cat have a thick white scab line on her back?

Almost 100% of the time, scabs along the back of a cat is due to a flea hypersensitivity. I would say this is more likely if more than one cats is involved. Cats do not have to go outdoors to get fleas. So, if you don't have them on a good quality flea prevention then this is likely to be the cause. You don't always see fleas on them. Here is a link to an article about skin issues in cats: ( https://www.purina.co.uk/cats/health-and-nutrition/symptoms-to-watch-out-for/cat-skin-problems ) The other most likely option would be a bacterial or fungal infection of the skin. I'm sure your vet will be able to sort this out for you. Hope this helps. Best wishes.

It could be that Ruby sustained a wound from trauma that has caused the scab, and I would monitor it for now for signs of infection. These signs include redness, swelling, warmth to the touch, and puss. Should any of those signs be seen, I would take Ruby into the vet for antibiotics. I would not put any topical treatment on it. Although quite safe on superficial wounds, cats groom and ingest most of the topical ointment you apply and therefore should be used only under direct veterinary supervision and if there are no other appropriate options. Many human ointments contain multiple other ingredients which are not necessarily safe to be swallowed. Hopefully, it should heal just fine on it's own.

If her hair is matted then she will need to have the hair trimmed with a pair of clippers. She could develop a skin infection if the matted hair is against her skin. You could also bathe her and see if that helps.

Scabs like this could be caused by allergies (food allergies, or an allergy to something in the environment), or a bacterial or fungal skin infection. If you're positive that he doesn't have any fleas, then I would recommend having him examined by a vet when you are able to. It sounds like he will probably need antibiotics and a short-term steroid or other anti-inflammatory medication to treat the problem, and the vet can also talk to you about options to prevent this issue from recurring in the future - such as a diet chance to a prescription hypoallergenic diet, or a daily allergy medication if needed.

I would recommend that you have the cat checked out by a vet to see what is causing the skin issue. If it bleeds with simple things like getting brushed, then there is something wrong. It could be related to parasites, allergies, autoimmune disease, infection, etc.

Ouch! Those are some major hot spots in that photo. I would recommend having Lili seen by your vet so the area can be clipped and cleaned and any medications such as pain or antibiotics can be given to help it heal a bit faster. I'd also keep Lili from scratching or pawing at it as this can introduce bacteria and more inflammation which can slow healing. If only Lili is affected and you're not noticing other issues like parasites, etc, it is possible there is an allergy underlying. She could be rubbing up against something causing the itchiness, or could have an ingredient allergy to something in her food (or a treat if the spots correspond with intermittent feeding of a treat or wet food). Other issues such as an underlying illness like an immune or metabolic issue can also cause scabbing and skin issues that affect only one cat in the household. A vet visit is a good option at this point both to treat the current scabs and to see if a food trial, allergy meds, or additional testing such as bloodwork to rule out any underlying health issues may be beneficial in finding the cause.

Poor cat. This looks wide enough where it should at the very least get a topical treatment. You can try an over the counter antibitoic ointment or spray from your local pet store. Ask a store representative as to what is available in your area. You should also clean this area with Betadine diluted 50:50 with warm water to help keep bacteria and infection at bay. If it doesn't seem to be healing well in the next few days, then a vet visit is warranted at that point. I hope that this heals up well soon! Best of luck moving forward. Take care. : )